Permission
Check with your condo association to see what types of alterations to your unit are permissible. Usually, the rules and regulations that dictate what you can't do to your condo have to do with external appearance and safety. Insulation plays a role in the fire resistance of a unit, which is why some condo boards may have regulations that limit what types of insulation you can add to your walls. Get permission before having any work done.
Loose-Fill Insulation
One good option for beefing up a poorly insulated wall in a condo is to have loose-fill insulation blown into the wall cavity. To do this, a contractor would come in and cut a minimally invasive hole in the wall. Then, he would use a blower to smoothly distribute loose insulation into the wall cavity, filling the empty spaces in the wall. Different types of loose-fill insulation are available, including cellulose and fiberglass.
Complete Retrofit
If permission can be obtained, you can also open up the walls in your condo and have them insulated. This method might be used to improve the insulation between your unit and the exterior of the building, or to install or improve insulation between your condo and neighboring units. At this point, you can choose from insulation options including simple batt insulation or an expanding foam product. Decide whether you're trying to focus on heat transmission or sound blocking, since different insulation products are designed for specific purposes.
Alternatives
In the event that you're not allowed to install insulation in the walls of your condo, alternatives can help reduce sound transmission and prevent heat loss. Installing insulating window films seals the largest source of heat loss in the room. Adding another layer of drywall or a specialty sound-insulating wall board blocks sound coming from other units.